1. Field of the Invention
The present invention relates to a color conversion definition method and a color definition apparatus that define a conversion from a coordinate point in a color reproduction range of a device (for example, printer) intervening between image data and an image to a coordinate point in a color reproduction range of a printing system, wherein the coordinate point in the color reproduction range of the device is expressed in a three-dimensional color space (RGB color space) having a R (red) axis, a G (green) axis and a B (blue) axis, dependent on the device (printer), and wherein the coordinate point in the color reproduction range of the printing system is expressed in a four-dimensional color space having a C (cyan) axis, a M (magenta) axis, a Y (yellow) axis and a K (black) axis. The present invention also relates to a color conversion definition program storage medium which stores a color conversion definition program that is executed on an information processing apparatus such as a computer thereby allowing the information processing apparatus to operate as the color conversion definition apparatus. The present invention also relates to a profile production method and a profile production apparatus that produce a profile that defines the correspondence between different color spaces. The present invention also relates to a profile production program storage medium which stores a profile production program that is executed on an information processing apparatus such as a computer thereby allowing the information processing apparatus to operate as the profile production apparatus.
2. Description of the Related Art
An apparatus is known that performs high-quality color processing on image data representing an image to be printed, such that when CMY data representing a combination of density values of C, M, and Y (a coordinate point in a CMY color space) is given, CMYK data representing a combination of dot percentage values of C, M, Y and K printing plates (a coordinate point in the CMYK color space) is output (an example may be found, for example, in Japanese Unexamined Patent Application Publication No. 9-83824, which will be hereinafter refereed to as Patent Document 1).
The technique associated with the apparatus has been established, although some improvement is still being made. There are many skilled persons who can operate the apparatus to perform high-quality color processing (referred to as setup).
In recent years, with increasing popularity of color management techniques, there has arisen a need for a technique of producing high-quality CMYK data intended for use in printing in accordance with color data other than CMY data. A specific example is a need for printing an image in accordance with given RGB data representing combinations of R, G, and B values (coordinate points in the RGB color space) such that the printed image has colors very similar to colors of an image printed by a particular printer in accordance with the same RGB data.
When RGB data is converted to CMYK data, it is required that the resultant CMYK data should represent a color that is calorimetrically identical to a color represented by the RGB data, and furthermore the resultant CMYK data should be excellent in printability. One of important factors associated with printability is a K value. When RGB data is converted to CMYK data representing a calorimetrically equal color, the K value has to be determined depending on a printing company or a printing system (in accordance with a printing K plate constraint (or a K-value constraint)).
Even if RGB data can be converted to CMYK data that represents a calorimetrically equal color and that is excellent in printability, using some technique, the color represented in an image printed by a particular printer in accordance with the RGB data becomes the same as the color represented in an image output in accordance with the CMYK data only a particular region in which the color reproduction range of the printer and the color reproduction range of the printing system overlap each other. When the color reproduction range (defined by edges of a printer profile) of the printer is greatly different from the color reproduction range (defined by edges of a printing-system profile) of the printing system (the color reproduction range of the printing system is generally narrower than the color representing range of the printer), a technique is necessary that can convert the RGB data to CMYK data that represents a color tone that is vary natural and very similar to the color tone of an image printed by the printer in accordance with the RGB data. Such conversion is referred to as gamut mapping.
An excellent gamut mapping technique has been proposed (Japanese Unexamined Patent Application Publication No. 2001-103329, which will be hereinafter refereed to as Patent Document 2). In the technique disclosed in Patent Document 2, the direction of mapping is determined in a color space intended for use by a device (for example, a RGB color space dependent on the device), but actual mapping is performed in a reference color space such as a L*a*b* color space. Use of this technique makes it possible to achieve high calorimetric accuracy in a region close to the gray axis and also achieve representation of high saturation colors in a region close to the boundary surface of the color reproduction gamut.
However, the technique disclosed in Patent Document 2 cannot directly map RGB data to CMYK data including a K value. Therefore, to map RGB data to CMYK data used by a printing system, for example as disclosed in Japanese Patent Application No. 2002-331112, the mapping is performed via an intervening device which deals with RGB data and which has a color reproduction range substantially equal to the color reproduction range of the printing system. That is, first, gamut mapping based on the technique disclosed in Patent Document 2 is performed from input RGB data to RGB data dealt with by the intervening device, and then the resultant RGB data is converted in terms of color matching to CMYK data used in printing in accordance with the K-value constraint. In this conversion technique disclosed in Japanese Patent Application No. 2002-331112, it is necessary to actually prepare the intervening device having the color reproduction range very similar to the color reproduction range of the printing system. To avoid this problem, Japanese Patent Application No. 2002-261174 discloses a technique to convert data via a virtual intervening device without preparing an actual device. That is, input RGB data is first converted (gamut mapped) to RGB data of the virtual intervening device having a color reproduction range very similar to that of the printing system, and the resultant RGB data is then converted in terms of color matching to CMYK data used in printing by the printing system in accordance with the K-value constraint.
However, Japanese Patent Application No. 2002-261174 simply discloses an idea of use of a virtual intervening device having a color reproduction range very similar to that of the printing system. However, actual conversion from RGB data intended for use by a printer to CMYK data for use by a printing system is not so simple. The color reproduction range of RGB data is given by a regular hexahedron with eight vertices in which (R, G, B) can vary from (0, 0, 0) to (255, 255, 255) (in this specific example, the maximum value of R, G, and B is 255). Therefore, when the color reproduction range of RGB data is mapped, for example, into the L*a*b* color space, the resultant range in the L*a*b* color space also has eight vertices. In contrast, the color reproduction range of CMYK data has additional vertices corresponding to black (C, M, Y, K)=(100, 100, 100, 100), reddish black (C, M, Y, K=(0, 100, 100, 100), greenish black (C, M, Y, K)=(100, 0, 100, 100), and bluish black (C, M, Y, K)=(100, 100, 0, 100) (where C, M, Y, and K represent in dot percentage values of C, M, Y, and K printing plates, and the maximum allowable value of dot percentage is 100), and thus the color reproduction range of CMYK generally has a greater number of vertices (typically eleven vertices) than the color reproduction range of RGB data. This means that the range mapped from the color reproduction range of RGB data dealt with by the printer is not the same as the color reproduction range of the printing system. Thus, it is necessary to well handle this difference. Furthermore, in the conversion technique disclosed in Japanese Patent Application No. 2002-261174, although the idea of use of a virtual intervening device having a color reproduction range very similar to that of the printing system is disclosed, no discussion is made on the specific profile of the virtual device. However, the applicability of the gamut mapping disclosed in Patent Document 2 greatly varies depending on the profile of the virtual device, and thus the definition of the profile of the virtual device is important.
In view of the above, an object of the present invention is to provide a color conversion definition method of defining conversion from a coordinate point (RGB data) in a color reproduction range of a device such as a printer, represented in a RGB color space dependent on the device to a coordinate point (CMYK data) in a color reproduction range of printing, represented in a CMYK color space of printing, such that RGB data dealt with by the device is converted to CMYK data that can be used in printing to obtain a printed image having colors very similar to colors of an image output by the device that deals with the original RGB data. Another object of the present invention is to provide a color conversion definition apparatus that defines conversion from RGB data to CMYK data according to the above method. Still another object of the present invention is to provide a color conversion definition program storage medium which stores a color conversion definition program that is executed on an information processing apparatus such as a computer whereby the information processing apparatus operates as the color conversion definition apparatus.
Another object of the present invention is to provide a profile production method of producing a profile that sufficiently well approximates the color reproduction range of printing without creating gray level distortion in gamut mapping. A still another object of the present invention is to provide a profile production apparatus that produces such a profile. It is a still another object of the present invention to provide a profile production program storage medium which stores a profile production program that is executed on an information processing apparatus such as a computer whereby the information processing apparatus operates as the profile production apparatus.
Another object of the present invention is to provide a profile production method of producing a link profile that defines the correspondence between RGB data and CMYK data such that a given K-value constraint is strictly satisfied on a gray axis and in a region close to the gray axis and such that high saturation colors can be represented. A still another object of the present invention is to provide a profile production apparatus that produces such a link profile. It is a still another object of the present invention to provide a profile production program storage medium which stores a profile production program that is executed on an information processing apparatus such as a computer whereby the information processing apparatus operates as that profile production apparatus.